While not limited thereto, the present invention is particularly adapted for use as a fast breeder reactor duct and fuel rod cladding alloy. Such an alloy requires strong mechanical properties at high temperatures and at the same time must have both swelling resistance under the influence of irradiation and low neutron absorbence. Alloys such as those described in U.S. Pat. No. 3,046,108 to Eiselstein, disclose age-hardenable nickel-chromium base alloys which have high strength and good ductility over a wide temperature range up to about 1400.degree. F. Specifically, the aforesaid patent discloses a nickel-base alloy having a nominal composition consisting essentially of about 53% nickel, about 19% chromium, about 3% molybdenum, about 5% niobium, about 0.2% silicon, about 0.2% manganese, about 0.9% titanium, about 0.45% aluminum, about 0.04% carbon and the balance essentially iron. The alloy is characterized in the age-hardened condition by a yield strength (0.2% offset) of at least 100,000 pounds per square inch at room temperature and by a 100-hour rupture strength of at least 90,000 p.s.i. at 1200.degree. F.
An article by R. Cozar and A. Pineu appearing in "Metallurgical Transactions", Vol. 4, January 1973, page 47, explains that nickel-base alloys containing titanium and aluminum, such as those described in U.S. Pat. No. 3,046,108 are strengthen by precipitation of a gamma-prime phase. It has also been found that by adjusting the amounts of titanium, aluminum and niobium in such alloys, a morphology can be obtained wherein precipitated gamma-prime particles are coated on their six faces with a shell of gamma-double prime precipitate. The resulting microstructure is very stable on prolonged aging and has thermal stability better than that encountered with most alloys described in U.S. Pat. No. 3,046,108.
While the mechanical properties at high temperatures of alloys such as those described above are particularly suitable for use in nuclear applications, they generally contain in excess of 50% nickel and in excess of 5% niobium, both of which act as neutron absorbers which makes them undesirable for breeder reactor applications. It is, therefore, desirable to employ an alloy which has reduced amounts of these alloying additions; but at the same time, it has been found that alloys containing about 37% nickel, for example, will not precipitate the gamma-double prime phase and that the ratio of atomic percent iron-to-nickel must be less than unity to give the requisite mechanical properties. Thus, the known alloys, while having the requisite mechanical properties, are deficient in one or more respects under the influence of irradiation as is encountered, for example, in a fast breeder reactor.